The effect of drugs on a fixed-ratio performance suppressed by a pre-time-out stimulus.

Pecking was reinforced by a fixed-ratio schedule with food, and responses during a red light produced a time out. If the bird did not respond during the red light, the light terminated and the bird could complete the FR schedule of positive reinforcement uninterrupted. The bird stopped responding during the red light sufficiently to avoid most of the possible time outs. In general, the pre-time-out stimulus suppressed responding more when the FR schedule was large than when it was small. The occurrence of the pre-time-out stimulus in the fixed ratio produced FR strain and extreme curvature atypical of normal fixed ratios of this size. Amobarbital, pentobarbital, chlorpromazine, and d-amphetamine injected when the FR performance was strained by the pre-time-out procedure produced marked increases in responding. The drug administration lowered the rate of responding only at larger doses; and then this occurred predominantly just after the injection.